Ash spews from Mexico's Popocatepetl volcano, about 35 miles from Mexico City on July 4, 2013. / Pablo Spencer, AFP/Getty Images

by The Associated Press, USA TODAY

by The Associated Press, USA TODAY

MEXICO CITY (AP) - U.S. airlines canceled more flights in and out of Mexico City's airport on Friday as a precautionary measure as the nearby Popocatepetl volcano continued to emit vapor and ash.

Alaska Airlines, United, Delta and AirTran canceled about a dozen flights, fewer than the number on Thursday, said Jorge Gomez, spokesman for Mexico City International Airport.

He noted that the airlines made the decision, and said normal operations continued at the airport without restrictions. No ash has fallen at the airport, Gomez said, though dust particles have been detected from the volcano that is about 40 miles (65 kilometers) away.

At least six U.S. airlines canceled more than 40 flights on Thursday as the volcano spewed a mile-high (1.5 kilometer-high) plume of ash that drifted over large parts of Mexico City. The volcano also spewed a hot shower of glowing rock around its crater.

Mexico's National Center for Disaster Prevention reported that there had been 99 tremors and exhalations of medium and high frequency from the volcano on Friday afternoon.

Activity has increased this week from the volcano that towers more than 15,000 feet (5,450 meters) high in central Mexico where the states of Mexico, Puebla and Morelos meet.

Ash has fallen on towns at the volcano's base and as far away as some neighborhoods in Mexico City.

The Environment Ministry called on residents to take preventive measures Friday against the falling ash, including wearing dust masks, covering water supplies and staying indoors as needed.

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